Digging Into Apple's Fusion Drive Details

By Dave Hamilton

Oct 23rd, 2012 4:11 PM EDT

One of the more interesting things released today at Apple's event was the new "Fusion Drive" in the new iMac (and yes, it's only for the new iMac and new Mac mini models so far). Fusion Drive is Apple's take on solving the "I want the speed of an SSD but I have way more stuff than I can fit on the SSD I could afford" problem.

Apple's Fusion Drive Performance Chart

In a nutshell, Fusion Drive combines TWO drives, a solid state drive (SSD) and a mechanical drive. Apple's initial incarnation puts a 128GB SSD in with either a 1TB or a 3TB mechanical drive. This is not a simple RAID, however, as 100% of the "magic" is done within OS X itself. What Fusion Drive does is it watches what files and applications you run most regularly and it moves them to the SSD. It will also take stuff that you aren't using all that often and moves it to the mechanical drive.

Fusion Drive

Not a Cache

To be clear, this is not a caching concept, at least not in the current use of the word. Cache would imply that the data on the SSD is duplicated, and it's not. If you have a 1TB mechanical drive paired with the 128GB SSD, you have a 1.12 TB storage platform. This truly is the fusion of all the space on two separate disks.

Not a RAID

As I said before, this is all being done at the level of the operating system. Disk Utility sees two physical disks, and the OS simply marries them together with Fusion Drive as a layer on top. It does announce just one disk to the user (and to Applications), which means the OS does the management and you don't have to worry about it (but you should still back up).

Phil Schiller Demonstrates Fusion Drive Concept

What About Backups?

Well, Time Machine will back this up as one volume. If you lose one of your drives (the SSD or the mechanical) you would need to do a full restore, according to Apple. But if you don't have a backup, this setup (unlike a RAID) means you could go in and extract files from the working disk, though I would expect that to be a bit of a chore. Totally do-able, though.

On the subject of replacement drives, Apple did indicate that there's nothing special (or, well different) about the drives in this iMac, so if your 3TB drive failed and you wanted to replace it you could, in theory, do so with 3rd party stuff. You'd have to reconfigure Fusion Drive, and it should work, but of course Apple's not going to support that.

Also Benefits Writes

Since the set up is already there, Apple added a v1.0 iteration (something I love): all write operations are first saved to the SSD. Then, of course, if the OS decides they belong better on the mechanical disk, that data will be moved at another time. But the benefit is that writes happen really fast because they're all going to the SSD.

Sounds Good, Right?

Yes, it does sound good. This may be one of the biggest announcements of the day. There are other hybrid models of blending SSD and mechanical disks to save money, but none of them are able to integrate with the OS and do it as intelligently as Apple is able to with Fusion Drive.

The real benefit, of course, is dependent on how Apple has set up its algorithms to manage all this. But we know how Apple is: it'll be pretty solid out of the gate and then, assuming it's something the company wants to pay attention to, it will get remarkably better over the next 6 to 12 months, too.